Recently, the MPEG (Moving Picture Experts Group) standard, i.e., an international standard of media integration type moving picture compressing technique capable of compressing many informations, have been widely used according to development of multimedia techniques integrating audio, data and video.
MPEG techniques capable of compressing many informations are applied to a communication, broadcasting, home appliances, computer, CD-ROM, etc., to synchronize the audio and video, thus multiplexing the audio and video.
Generally, a bitrate of MPEG bitstream data, which is a system stream mixed with the audio and video signals, is determined according to a kind of channel and memory or display. Such a bitstream data is received to the MPEG decoder and first, decoding of the system stream is executed to obtain audio bitstream informations, video bitstream informations and additional informations mixed into the system stream.
The prior art MPEG decoder decodes MPEG bitstream data to write the decoded results in an external memory. Then, the decoder converts the decoded results into analog signals so that we can hear the audio or see the video.
Generally, since each platform has different computational power, it is necessary to provide the maximum quality of the services which is achievable with the capabilities of each platform. And also, it is mandatory that all decoders can decode all MPEG-4 services but at different levels of quality. It means that a scalability based on computational power is needed. The scalability should enable a graceful degradation of the quality when less than theoretically necessary computational power is available at decoding side.
The conventional computational graceful degradation method has been proposed in MPEG96/0925 by M. Mattavelli et al. to reduce the amount of computation when decoding the encoded multimedia data. The method decreases, in a lump, the amount of computation necessary for decoding without consideration of relative importance between the respective video objects if computational power of the decoder does not come up to that necessary for processing the encoded multimedia data. In the conventional method, however, there exists a problem which does not make full use of the relative importance between the respective video objects.
The conventional method has considered only a single video object (VO) case. A video scene, however, is composed of multiple VOs which can be further decomposed into several video object layers (VOLs) in order to allow scalability. The various degree of visual importance of these VOs and VOLs should be exploited for the simplified decoding in the context of computation graceful degradation.